The idea behind observing the home planet is to give scientists an understanding of how an alien Earth may look in the data collected by future telescopes, chemical-analyzing spectrographs and other instruments.

"It's basically an extra tool to have in our tool belt as we go looking for exoplanets," said Nicolas Cowan, with the University of Washington.

Cowen and colleagues have been using NASA's Deep Impact spacecraft to study Earth from millions of miles away. Originally dispatched to blast a hole into comet Tempel 1 so scientists could study the comet's interior, Deep Impact was given a follow-on assignment to visit comet Hartley 2 in November 2010 and search for extrasolar planets along the way.

Cowen focused on understanding how an ocean-bearing world like Earth, with continents interspersed with water, would impact the overall reflectivity of light in seven select wavelengths, particularly those in the near-infrared.

They found that as Earth completed a 24-hour rotation, the change in brightness varied by about 30 percent up and down. The shift is caused by the planet's reflectivity as the sun alternatively shines on oceans and then continents.

"The key is to look at how the planet changes over time," Cowen told Discovery News.

Clouds were not found to be a significant contributor to the change in brightness because they typically do not completely dissipate or form within a 24-hour period.

"You could imagine some weird planet where the clouds change very quickly and then you wouldn't be able to use this technique. You'd be hosed," Cowen said.

Another show-stopper would be if a planet's water were in one hemisphere and its mass of land on the other, as the total amount of light reflected from its parent star would stay stable throughout a rotation.

Cowen says that variable brightness — particularly in the wavelengths where light reflected off water and land show up most vividly — might be an important clue that an alien planet shares Earth's proclivity for water, and perhaps life.

"The spectral resolution and coverage we used will be close to optimal for Earth-like planets," Drake Deming, the spacecraft's deputy principal scientist, wrote in an e-mail to Discovery News.

Follow-up investigations to assess the chemical makeup of a target planet's atmosphere and its surface composition would be needed before any definitive claim could be made that an alien planet has oceans.

"There's going to be a lot of skeptics if you go and claim you found water on an extrasolar planet," Cowan said.

Scientists plan to use Deep Impact to get some polar views of Earth before the spacecraft heads out for its comet rendezvous.

"We imaged the Earth from above the north pole in March, and we are looking forward to a south pole view in September," said Deming.

Cowan's results will be published in the August issue of the Astrophysical Journal.